This invention relates to an illumination apparatus in which a light emitting diode element is used and an image display apparatus which uses an illumination apparatus of the type described as a light source.
In a display device such as, for example, an LCD (Liquid Crystal Display) device, a light source (illumination apparatus) called backlight is used to display an image by visible radiations. In recent years, an illumination apparatus has been proposed wherein a light emitting diode (LED) element (hereinafter referred to as “light emitting diode”) is used for the backlight. Where a light emitting diode is used for a backlight, usually a technique wherein light emitting diodes which emit lights of the primary colors of red (R), green (G) and blue (B) are used and lights obtained by causing the light emitting diodes to emit light are optically mixed by additive synthesis to obtain white light is adopted.
In order to actually drive such a light source in which light emitting diodes corresponding to the primary colors of R, G and B as described above are used so that the light source actually emits light, it is a possible idea to adopt a configuration similar to that of a matrix driving system which is used to display an image.
According to the matrix driving system, pixels are disposed in a matrix along an X direction (horizontal direction) and a Y direction (vertical direction), and suitable electrodes are disposed along the X direction (horizontal direction) and the Y direction (vertical direction) in a corresponding relationship to the pixels. Then, the electrodes are driven at required timings based on a setting of the pixels to be driven to emit light and a setting of timings at which the pixels are to be driven to emit light. It is to be noted that the driving timings at this time differ depending upon the difference of the driving system within the criterion of the matrix driving system. The pixels are driven in this manner to represent, for example, gradations for the individual pixels to display an image of visible radiations on an entire screen.
However, the driving circuit system of such a matrix system as described above is complicated and requires a high cost. Particularly with illumination applications to a backlight or the like, the power consumption by the light emitting diodes is comparatively high. Under present conditions, parts such as an LSI for implementing matrix driving which are ready for such high power driving as described above are little available. From such a situation as just described, it is considered impractical to adopt the driving circuit system of the matrix system as a configuration for driving the light source.
Where a backlight is formed using light emitting diodes corresponding to the primary colors of R, G and B, the difference in light emission efficiency, voltage drop, power consumption and so forth between the light emitting diodes of the colors of R, G and B matters. The semiconductor composition of a light emitting diode differs among different colors, and this appears as such a difference in characteristic among the light emitting diodes as described above. Therefore, it is considered that, in order to obtain good white color, it is advantageous to drive the light emitting diodes of the colors of R, G and B independently of each other and adjust the light quantity for each of the colors.
From such a background as described above, in order to drive a backlight formed using light emitting diodes corresponding to the three primary colors of R, G and B, the following basic configuration is adopted popularly.
First, as a minimum unit of a block of a backlight, a light emitting diode cell 100 is provided in such a manner as seen in FIG. 18A. The light emitting diode block or cell is formed by preparing a predetermined number of light emitting diodes of predetermined colors, disposing the light emitting diodes at predetermined positions of a board or the like and electrically connecting the disposed light emitting diodes in accordance with a predetermined pattern. In the light emitting diode block shown in FIG. 18A, totaling six diodes are prepared including two red light emitting diodes DL-R corresponding to R (red), green light emitting diodes DL-G corresponding to G (green) and two blue light emitting diodes DL-B corresponding to B (blue) The light emitting diodes are disposed in order of blue-green-red-blue-green-red from the left side to the right side as seen in FIG. 18A. Further, the light emitting diodes of each color are connected in series with the same polarity.
It is to be noted that light emitting diode elements in an LED cell may possibly be disposed in different disposition patterns. The disposition pattern of light emitting diode elements is determined, for example, in response to the rated specifications, light emitting efficiencies and so forth of light emitting diodes to be used actually so that white light of a good quality is obtained as mixed color of R, G and B.
The light emitting diode cell 100 formed in this manner can be connected to another light emitting diode cell 100 of the same type with regard to both of the anode side and the cathode side of the series connections of the light emitting diodes of the individual colors of R, G and B as seen in FIG. 18A. Where the light emitting diode cells 100 are connected to each other in this manner, the number of series connections of the light emitting diodes corresponding to each of the colors of R, G and B increases in accordance with the number of LED cells connected.
Thus, a required number of light emitting diode cells 100 are connected to form a block. As a particular example, three light emitting diode cells 100 are connected to form one block in FIG. 18B. Here, the block is referred to as LED cell block 101. Since one light emitting diode cell 100 includes two light emitting diodes for each of the colors of R, G and B, the number of colors of the light emitting sources of the light emitting diode cell 100 is represented as (2R, 2G, 2B). In the arrangement of FIG. 18B, since the LED cell block 101 is formed from three light emitting diode cells 100, the number of colors can be represented as 3 (2G, 2R, 2B)=(6G, 6R, 6B).
Then, such LED cell blocks 101 formed in such a manner as described above are disposed on a plane to construct, for example, a panel having a function as a backlight. An example of a backlight panel 110 formed using the LED cell block 101 shown in FIG. 18B is shown in FIG. 19.
Referring to FIG. 19, the LED cell blocks 101 are disposed in a matrix of 5 rows×4 columns including rows g1 to g5 and columns m1 to m4 to form the backlight panel 110. The backlight panel 110 includes a total of 6×5×4=120 red light emitting diodes DL-R. Similarly, the backlight panel 110 includes totaling 120 green light emitting diodes DL-G and 120 blue light emitting diodes DL-B. Consequently, the backlight panel 110 includes totaling 360 (=120×3) light emitting diodes.
As described above, it is considered impractical to perform light emission driving of a large number of light emitting diodes, which emit lights of the different colors of R, G and B, in accordance with the matrix driving system in this manner so that good white light may be obtained, and under present conditions, driving, for example, by such a method as described below is used popularly.
FIG. 20 illustrates a concept of a configuration for driving the light emitting diodes of the backlight panel of the structure shown in FIG. 19. Referring to FIG. 20, the LED cell blocks 101 which form the backlight panel are connected such that the LED cell block 101s in each of the rows g1 to gn are connected to each other in a horizontal direction. Consequently, in each of the rows g1 to gn, the light emitting diodes corresponding to each of the colors of R, G and B are connected in series in order of the columns m1 to mn.
For the light emitting diodes connected in such a connection scheme as described above, three DC-DC converters 120-R, 120-G and 120-B corresponding to the colors of the R, G and B are provided for each of the rows g1 to gn. Then, an output of the DC-DC converter 120-R is connected to the anode side terminal of the series connection circuit of the red light emitting diodes DL-R (that is, to the connection position on the anode side of the LED cell block 101 positioned in the column ml). Similarly, the DC-DC converters 120-G and 120-B are connected to the anode side end portion of the series connection circuit of the green light emitting diodes DL-G and the anode side end portion of the series connection circuit of the blue light emitting diodes DL-B, respectively.
In the configuration described above, DC driving current is supplied from a DC power supply outputted from the DC-DC converter 120-R to the red light emitting diodes DL-R connected in series along one row to drive the red light emitting diodes DL-R to emit light. Similarly, the green light emitting diodes DL-G connected in series along the same row are driven from a DC power supply outputted from the DC-DC converters 120-G to emit light. Further, the blue light emitting diodes DL-B connected in series along the same row are driven by a DC power supply outputted from the DC-DC converters 120-B to emit light. Such a configuration of a driving circuit system as just described is formed for each row.
FIG. 21 shows an actual configuration of a drive circuit for a series connection circuit of light emitting diodes. Referring to FIG. 21, a DC voltage Vcc which is an output of a DC-DC converter 120 is applied to the anode side terminal of an LED series circuit 130 formed from light emitting diodes connected in series so as to interpose a resistor R42. Consequently, driving current ILED flows through the light emitting diodes DL which form the LED series circuit 130.
Further, the DC-DC converter 120 performs constant current control such that it detects a voltage drop of the predetermined DC voltage Vcc across the resistor R42 at a predetermined timing and performs constant current control so that the driving current ILED to flow may be fixed. For the constant current control, a resistor R41, a capacitor C41, a switching transistor Q12 and a sampling timing production/switch driving circuit 131 are provided additionally. The sampling timing production/switch driving circuit 131 in this instance generates a sample hold timing based on a PWM signal (rectangular waveform signal) inputted thereto through an AND gate 132 and on/off controls the switching transistor Q12 which functions as a sample hold switch. Consequently, the DC-DC converter 120 detects a voltage drop across the resistor R42 at the sample hold timing. The DC-DC converter 120 performs constant current control of the power to be supplied as the DC voltage Vcc in response to the detected voltage drop level. Further, a control section (CPU) 140 controls a level shift circuit 141 in response to a result of detection of a sensor 142 which detects, for example, the temperature so that a reference level Lref to be used for the constant current control by the DC-DC converter 120 can be varied. Consequently, an appropriate constant current amount corresponding to a temperature variation can be obtained for the driving current ILED.
Further, a PWM signal supplied from a driver not shown is used to perform on/off control of a transistor Q11 in a period of the PWM signal to control continuity/discontinuity of the driving current ILED. Consequently, the continuity time of the driving current ILED per unit time is controlled in response to the pulse width of the PWM signal within one period. In other words, the amount of light to be emitted from the light emitting diode can be controlled. Further, an output of the AND gate 132 to which the PWM signal and the on/off signal are inputted is applied to the gate of the transistor Q11. In particular, the light quantity control (and constant current control) of the light emitting diode described above can be set between on and off by changeover of the on/off signal between the H (High) level and the L (Low) level. The on/off signal is outputted, for example, from the control section 140 which performs changeover between the H level and the L level in response to an operation situation and so forth.
FIG. 22 shows a configuration of a control loop for the light quantity control of light emitting diodes described above. It is to be noted that, in FIG. 22, like elements to those in FIG. 21 are denoted by like reference characters and overlapping description of the common elements is omitted herein to avoid redundancy.
Referring to FIG. 22, a photosensor 150 detects a light quantity of a light emitting diodes DL which form the LED series circuit 130 as a current amount and outputs the detected current amount to an I-V amplifier 151. The I-V amplifier 151 is an amplifier formed from an operational amplifier OP, a resistor R31, a capacitor C31, another resistor R32 and another capacitor C32 connected in such a manner as seen in FIG. 22. The I-V amplifier 151 operates so as to convert a current amount inputted thereto into a voltage value. The analog voltage value outputted from the I-V amplifier 151 is converted into a digital value by an A/D converter 152 and inputted as information of a detected light quantity value to the control section 140.
The control section 140 refers to light quantity control data stored in a memory 153, for example, of the nonvolatile type to acquire a control value corresponding to the detected light quantity value inputted thereto and controls a driver 154 with the control value. The driver 154 varies the pulse width of the PWM signal with the control value and applies the PWM signal of the varied pulse width to the transistor Q11. Consequently, an appropriate light quantity is obtained and the emitted light quantity of the light emitting diodes DL is variably controlled. Such light quantity control is performed in order to maintain, for example, appropriate white light. In short, the emitted light quantity of the light emitting diodes corresponding to the individual colors of R (red), G (green) and B (blue) are controlled so that the emitted light quantity of the colors of R (red), G (green) and B (blue) are well balanced to obtain appropriate white light. Since the light emission efficiency of the light emitting diodes differs depending upon the color of the emitted light as described hereinabove, it is considered that, under present conditions, it is appropriate to perform light quantity control of the light emitting diodes using control loops independent of each other for the individual colors in such a manner as described above.
It is to be noted that related apparatus are disclosed, for example, in Japanese Patent Laid-Open No. 2001-272938 and Japanese Utility Model Laid-Open No. Sho 63-64059.
The configuration of the illumination apparatus as a backlight shown in FIGS. 18A to 22 can be formed in a circuit scale suppressed when compared with an alternative configuration wherein driving, for example, according to a matrix driving method is used. However, the configuration is still obliged to have a proportionately large circuit scale.
For example, according to the configuration, a DC-DC converter is used to obtain DC current in order to supply power for driving light emitting diodes as seen in FIG. 20. Where a light emitting diode is used for the illumination, proportionately high power is required, and therefore, such a countermeasure is taken that a number of DC-DC converters, for example, suitable for a series connection circuit of light emitting diodes are provided to achieve stabilized light emitting operation. In other words, a comparatively great number of DC-DC converters are required, and this makes reduction of the circuit scale difficult. A DC-DC converter includes a large-size part such as, for example, a transformer.
Further, provision of such a large number of DC-DC converters also increases the total power loss of the DC-DC converters and accordingly is disadvantageous in terms of the power consumption.
Further, as seen from FIGS. 21 and 22, also the control circuit system for performing the light quantity control, constant current control and so forth of the light emitting diodes must be provided for each of series connection circuits of the light emitting diodes. Also this makes a factor of obstructing reduction of the circuit scale.
In this manner, also under present conditions, reduction of the circuit scale of an apparatus wherein a light emitting diode is used for a light emitting source for illumination remains at a certain level, and it is demanded to form an apparatus of the type described in a simpler configuration of a reduced scale.
It is desirable to provide an illumination apparatus and an image display apparatus which are configured simply in a reduced scale while a light emitting diode is used as a light emitting source for illumination.